This invention lies in the field of wire spring assemblies such as those which form the foundation of a box spring used for supporting a mattress. It is directed to the construction and interconnection of components and more particularly to the construction of coil springs having attachment heads which are shaped for detachable connection to a wire grid by flexing movement without the use of special tools and resiliently grip the grid to lock them in place and prevent rotational movement.
The usual commerical methods of building box springs include building a mesh or wire grid having a wire border member and connectng individual coil springs to the grid, connecting the assembly to a base, and applying suitable padding and covering material. The springs are connected to the grid by welding, tying, wire twisting, clipping etc. while connections of this type have generally produced rather satisfactory spring assemblies they are subject to loosening, except for welded connections, and wear resulting in failure. In addition, they all require special machinery or tools and a great deal of hand labor all of which in recent years have added greatly to the cost of manufacture.
To overcome these disadvantages various schemes have been proposed for connecting springs to a mesh or grid without extraneous devices by distorting the coils or the grid wires to interengage resiliently and to secure the coils in position by elastic opposing forces between the interengaged parts. Examples of such schemes may be found in such U.S. Pat. Nos. as 1,871,440 to Barnard, 3,660,854 to Garceau, 3,761,972 to Bihun et al, and 3,766,578 to Toupal. While these various schemes greatly decrease the manual labor involved and hence improve the total manufacturing cost picture, they also introduce new problems and disadvantages.
In one class of such improved structures of grid wires are distorted or offset in various ways to provide seats or nests for the circular ends of the spring coils. This requires a great deal of machine forming work on the grid wires, and the formed portions must be very accurately located in the grid to cooperate with each other and produce correct nests in the proper places. Since the spring coil ends are circular it is possible for them to rotate even though elastic gripped. Changes in position lead to wear and failure.
In another class of such improved structures, the crossing grid wires are not positively secured to each other, as by welding, but spring coil ends are specially formed to engage both wires at a crossing point, both to secure the spring to the grid and to tie the wires together at the crossing point. The limited elastic grip cannot positively lock the wires together, and varying and repeated loads cause them to slide on each other, producing wear and displacement from their design locations. The spring coils themselves are not centered under the points of load application. The eccentric loading makes the coil less than fully effective, and tends to distort it out of shape and cause early breakdown.